1. Field of the Invention
The present invention relates to a zoom lens system that is suitable mainly for use as an imaging optical system for an image sensor, and includes a wide angle range of a half angle-of-view of 35 degrees.
2. Description of Related Art
In recent years there has been an increasing need for zoom lens systems in compact cameras to achieve a wider angle-of-view and a high zoom ratio. Whereas, further advancement in the miniaturization of the pixel pitch has occurred, thereby increasing the demand for the optical system to have a higher resolution. It is very common for photographing lens systems which are mainly used for compact cameras to achieve a slimmer camera body by employing a retractable mechanism that uses multi-stage barrels to reduce the air-distance between lens groups when the zoom lens system is fully retracted to the accommodation position. A zoom lens system that is suitable for such a retractable mechanism is required to have thin lens groups (small in the optical axis direction) as well as the frontmost lens group having a small lens diameter. Furthermore, at the design stage, consideration needs to be given to mechanical restrictions in regard to the overall length of the zoom lens system.
Zoom lens systems configured of a positive first lens group, a negative second lens group, a positive third lens group, and a positive fourth lens group, in that order from the object side, i.e., four lens groups, are known to be used for compact cameras (Japanese Unexamined Patent Publication Nos. 2006-23529, 2006-308957 and 2008-112013).
However, in the zoom lens system taught in Japanese Unexamined Patent Publication No. 2006-23529, the zoom ratio is approximately 5:1, and the angle-of-view at the short focal length extremity is only 61 degrees, and hence, does not achieve a sufficiently wide angle-of-view nor a sufficiently high zoom ratio.
Furthermore, in the zoom lens system taught in Japanese Unexamined Patent Publication No. 2006-308957, the angle-of-view at the short focal length extremity is approximately 78 degrees, achieving a wide angle-of-view; however, the zoom ratio is within a range of approximately 4:1 through 7:1, which are not satisfactory valves.
Furthermore, the zoom lens system taught in Japanese Unexamined Patent Publication No. 2008-112013, large amounts of astigmatism and chromatic aberration occur, and such aberration levels are unsatisfactory for a zoom lens system that is compatible with high pixelization.